1. Field of the Invention
The present invention generally relates to a procedure, operating method, and apparatus for autonomously providing a common time reference for device-to-device (D2D) communication of a wireless communication terminal.
2. Description of the Related Art
Wireless mobile communication systems in use today are mainly based on a wired network, and wireless communication is actually applied to a link between a base station and a terminal. Thus, if the wired network, on which a wireless mobile communication system is based, is damaged when various disasters occur, it is impossible to provide normal wireless communication services. In such emergency conditions, device-to-device (D2D) wireless communication may be used as one way to provide backup wireless communication services for the existing wired network-based wireless communication services.
The 3rd Generation Partnership Project (3GPP), the standards group for asynchronous cellular mobile communication, is also studying the support for D2D communication while conducting Long Term Evolution (LTE) release 12 standardization. In particular, 3GPP is currently under research and development and standardization aimed at D2D communication between LTE terminals, which supports proximity-based applications and services for terminals at a relatively long distance, as well as emergency backup communication services between terminals.
In order to achieve D2D communication, initially, a process of recognizing the existence of other terminals is required, which is referred to as a discovery process. Each D2D terminal transmits a discovery signal to notify other terminals of its existence. Further, each D2D terminal receives discovery signals transmitted from other terminals to recognize the existence of other terminals. In order to efficiently perform such an operation, a specific time interval where D2D terminals transmit/receive discovery signals must be predefined under the same common time reference. Otherwise, D2D terminals cannot know when neighboring D2D terminals transmit discovery signals, and thus have to always monitor discovery signals, which results in an increase in power consumption of the terminals.
In order to efficiently perform a discovery process for D2D communication, a common time reference has to be provided to D2D terminals. When a cellular network is normally operated, the common time reference may be acquired from a synchronization signal transmitted from a base station.
However, when a cellular network cannot be normally operated due to natural disasters and the like, or when a terminal is located out of the coverage area of the cellular network, there is a problem in that each D2D terminal cannot acquire common time synchronization from a base station.
Accordingly, there is a need for a way to provide a common time reference to D2D terminals. There is also a need to consider a method for providing a common time reference not by a base station but by a D2D terminal.